Recently, the traditional view that somatic stem cells remain faithful to their tissue of origin has been challenged. The capability of muscle-derived, and neural-derived cultures to produce cells capable of hematopoietic reconstitution following transplant, as well as the ability of hematopoietic cultures to give rise to muscle cells raises profound questions about the plasticity of somatic cells. The possibility that such plasticity might be exploited for therapeutic potential is compelling and makes a better understanding of its nature and the ways in which might be regulated of great interest. Plasticity, or the ability of cells that we think of as being lineage committed, to generate an alternative tissue or cell type must require the cells to reprogram and take an alternative developmental path. This must be true even of the stem cell populations within a tissue. An understanding of the basic mechanisms that define the pluripotent and plastic state somatic stem cells is the focus of this proposal. Our hypothesis is that the plasticity of stem cells a function of the translational repression of critical transcription factors present in the pluripotent stem cell. Plasticity, we also hypothesize, involves inhibition of expression of lineage defining transcripts already present within a cell resetting the developmental program along an alternative developmental pathway. To date, a proteomic approach to defining gene expression has not been undertaken. We have explored a novel and robust method for defining the change in translation upon the differentiation of stem cells. Using this approach we will be able to identify and study transcripts that are translationally repressed in the plastic/pluripotent state and gain an understanding of the basic mechanisms that underly plasticity and pluripotency. Specifically we will 1) identify critical translationally regulated mRNA transcripts and define an accurate picture of overall gene expression in the pluripotent cell by quantitating polysome-associated transcripts and their translational state using a rapid throughout screening assay in an in vitro model of stem cell plasticity; 2) define the mechanisms of translational regulation in transcripts critical to plasticity and pluripotency; and 3) define the role of translational regulation in modulating and maintaining pluripotency.